The annual flu shot is a notoriously tough sell, but what if the shots prevented heart attacks, too?

New Canadian research suggests that the influenza vaccine significantly lowers the risk of heart attack, strokes and dying from heart disease.

Toronto researchers who searched the medical literature back to the 1960s found that flu shots are associated with about a 50-per cent reduction in the risk of having a major cardiovascular "event" in the year following vaccination, for people with and without known coronary artery disease.

"It's a pretty profound finding," said Dr. Jacob Udell, a cardiologist and clinician scientist at Toronto's Women's College Hospital and the University of Toronto. Udell was scheduled to present his team's findings Sunday at the 2012 Canadian Cardiovascular Congress meeting in Toronto.

In a related study, another Toronto group found that people with implantable cardiac defibrillators - tiny pacemaker-like devices that deliver electrical jolts when needed to shock the heart back into a normal rhythm - appear less prone to having to be "shocked" during flu season if they are vaccinated against the flu.

Researchers say the findings, if proven in further studies, could boost stubbornly low vaccination rates against influenza. According to an Ipsos Reid survey published earlier this year, 36 per cent of Canadians reported having received a flu shot in 2011.

Exactly how flu vaccinations might lower the risk of heart attack or dying from heart disease isn't known. But Udell says there are several plausible explanations.

"Most people don't necessarily die of overwhelming pneumonia from the flu, but (rather) mostly from complications of the flu," Udell said.

Respiratory infections can affect the flow of oxygen through the blood and, by extension, to the heart.

Vaccinating people against the flu might also keep plaque deposits from bursting in an artery. With atherosclerosis, or hardening of the arteries, cholesterol and other substances build up in the walls of arteries. An infection with flu causes inflammation in the body - inflammation that can cause plaque to burst, blocking blood supply to the heart and leading to a heart attack.

"If you have a vaccine that's protecting you from an infection, it's technically protecting you from all the other bad things that come with infection, including inflammation," Udell said.

For their study, Udell's team pooled the results from four randomized controlled trials, from the Netherlands, Argentina, Poland and Thailand, conducted between 1994 and 2008. In total, 3,227 patients were involved. About half had established heart disease, while the other half had no evidence of cardiac disease.

According to the review, influenza vaccine significantly reduced the one-year risk of having a non-fatal heart attack or stroke, or dying from sudden cardiovascular disease.

The data not only sup-ort current guidelines that recommend flu shots for heart disease patients, influenza vaccine might be a potential therapy to reduce the risk of heart attack and stroke "with one simple yearly inoculation," according to the researchers.

Udell said it's not know if the findings would hold true for Canada. "But, even if it was a 10-per-cent reduction in heart attacks and strokes and dying from cardiovascular disease, that would have a profound impact on our population," he said.

An estimated 70,000 heart attacks occur in Canada each year, according to the Heart and Stroke Foundation of Canada. Between 2,000 and 8,000 Canadians die of flu and its complications annually, depending on the severity of the season, according to the Public Health Agency of Canada.

"It's well-known that there are more cardiac events that happen around winter time, for whatever reason," said cardiologist Dr. Sheldon Singh, of the Schulich Heart Centre at Sunnybrook Health Sciences Centre in Toronto.

For their study on patients with implantable defibrillators, Singh and his co-author, Dr. Ramanan Kumareswaran, surveyed 230 patients with implantable cardiac defibrillators, or ICDs. About 80 per cent said they had received a flu vaccination in the previous year.

ICDs do two things: When they detect a fast heart rate, the devices either deliver a jolt of electricity or "pace," or increase the heart rate to restore normal rhythm.

The patients who received the flu vaccine experienced fewer shocks or "paces."

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